High-Temperature Axial Fans: Materials, Motor Cooling and EN 12101-3 Smoke Classes

A high-temperature axial fan is defined by two questions: how hot is the gas, and for how long must the fan keep running? The answers separate a continuous process duty, where the fan runs hot for years, from an emergency smoke duty, where it must survive one fire for a set time and be certified to do so. Each shapes the choice of materials, motor arrangement, and testing, and confusing them is a common cause of failed specifications.

Continuous Duty and Emergency Duty Are Different Problems

Continuous high-temperature duty. The fan handles hot air or gas for its whole working life, for example in drying, heat treatment, or hot process exhaust. The concerns are thermal expansion, material strength at temperature, lubricant and bearing life, and motor cooling.

Emergency smoke duty. The fan normally runs at ambient temperature or stands idle, and must operate at a stated high temperature for a stated time during a fire. It is certified as a product to a fire-performance standard.

State clearly which one you need, along with the gas temperature, the duration, and any restart requirement.

EN 12101-3: Smoke Extraction Classes

EN 12101-3 covers powered smoke and heat exhaust ventilators. Fans are classified by temperature and duration. A class such as F300-120 means the fan has been shown to operate at 300 °C for 120 minutes, and F400-120 means 400 °C for 120 minutes. Common classes include F200, F250, F300, F400, and F600, each with a stated duration, commonly 120 minutes. Suppliers publish these classes for both axial and centrifugal smoke fans.

Points to remember: – The class is a certified product rating, based on testing, not a generic claim about materials. – An emergency-rated fan is not automatically suited to continuous hot duty, and the reverse is also true. – The rating applies to the tested configuration, including motor and any drive. – Local building and fire regulations define which class and duration a project needs.

What Temperature Does to Airflow

Hot gas is less dense. At the same speed a fan moves the same volume of gas, but pressure and power fall in proportion to density.

Gas temperature	Density (kg/m³)	Ratio to standard	A fan developing 500 Pa at standard density develops
20 °C	1.204	1.00	500 Pa
100 °C	0.946	0.79	393 Pa
150 °C	0.834	0.69	346 Pa
200 °C	0.746	0.62	310 Pa
300 °C	0.616	0.51	256 Pa
400 °C	0.524	0.44	218 Pa

At 150 °C a fan produces about 69% of the pressure it would produce at standard density, so selection must be made from the density-corrected duty. The opposite problem occurs at start-up: if the gas is cold when the fan starts, density is high and the motor absorbs the most power. Size the motor for the coldest, densest condition the fan will see, including start-up. See fan laws and air density.

Motor Position and Cooling

In many axial fans the motor sits in the airstream and is cooled by it. That works when the gas is cool, but in hot gas the motor is heated instead of cooled, and its insulation limits are exceeded quickly. Options include:

• Motor outside the airstream, driving the impeller through a shaft, sometimes with a belt drive.
• Cooling arrangements, such as an air-cooled shaft housing, cooling fins, or a supply of separate cooling air.
• Higher insulation class motors rated for the actual temperature, with thermistors for protection.
• Reduced load where the motor must be derated.

Bearings also need attention. Grease breaks down at temperature, so bearings are normally kept outside the hot zone or cooled, and lubricant is chosen for the temperature.

Materials

Material strength falls as temperature rises, and materials expand.

• Aluminium is light and common in ventilation impellers. Its strength decreases with temperature, and manufacturers commonly limit aluminium spark-resistant construction to roughly 120 to 135 °C (250 to 275 °F).
• Carbon steel suits higher temperatures and abrasive duty, with attention to oxidation and coatings.
• Stainless steel suits higher temperatures and corrosive conditions, and costs more.
• Coatings and paints have their own temperature limits, so a coating rated for ambient conditions can fail in hot gas.

Thermal expansion changes clearances. The gap between blade tips and casing must be set so that it stays safe when the fan is hot and does not open so far when cold that performance suffers.

Standard Ratings and Limits

Standard-construction fans are typically rated for ambient temperatures in a range of roughly −20 to +40 °C, as shown in some manufacturers’ specifications. Hot-gas capability is an option that has to be engineered and stated. Never assume a standard fan tolerates a hot gas stream because it is made of metal.

Restart and Cool-Down

After a hot shutdown, a fan that stops with the shaft still hot can bow, and bearings can suffer. Some installations use a cool-down period or a turning gear. State whether the duty involves frequent starts, standby operation, or automatic restart after a power dip.

Where Our Range Fits

We manufacture axial fans with impeller diameters up to 1,600 mm, static pressure up to 2,400 Pa, and air volume up to 185,000 m³/h. High-temperature duties, including motor-outside-airstream arrangements and special materials, are confirmed case by case, and smoke-extraction certification is a separate matter that we would confirm with you before you specify it.

What to Provide

1. Gas temperature, normal and peak.
2. Continuous or emergency duty, and duration.
3. Gas composition, dust, and moisture.
4. Required smoke class and standard, if any.
5. Motor arrangement preferences.
6. Start-up conditions, including cold starts.
7. Site altitude and ambient temperature.

Tell Us Your Gas Temperature and Duty

Send your temperature, duration, and duty type, and we’ll confirm what we can supply for a hot-gas or emergency application. Contact us for a free quote.

Frequently Asked Questions